Eclipse

World of Earth Science
COPYRIGHT 2003 The Gale Group, Inc.

Eclipse

An eclipse is a phenomenon in which the light from a celestial body is temporarily obscured by the presence of another.

A solar eclipse occurs when the Moon is aligned between the Sun and Earth. The trace of the lunar shadow (where the solar eclipse is visible) is less than 270 km (168 mi) wide. A partial eclipse is visible over a much wider region. When the Moon is further away from Earth, the lunar disc has a smaller visible diameter than the solar disc, so a narrow ring of the Sun remains uncovered, even when the three bodies are aligned. This produces an annular solar eclipse. The ratio between the visible lunar and solar diameters is called the magnitude of the eclipse. At the beginning of the solar eclipse, the Moon progressively covers the solar disk. Illumination of Earth's surface rapidly diminishes. The air temperature falls a few degrees. Seconds before the totality of the eclipse, shadow bands appear. Shadow bands are irregular bands of shadow, a few centimeters wide and up to

a meter apart, moving over the ground. The diamond ring phase of the eclipse then shines for few seconds and later, Bailey's beads appear on the solar limb. Bailey's beads are a string of bright beads of light produced by the uneven shape of the lunar limb.

In the first two to three seconds of the total phase of the eclipse (totality), the chromosphere is visible as a pink halo around half of the limb. Maximal duration of the totality varies from eclipse to eclipse, up to 7.5 minutes. The brightest stars and planets are observable on the sky during the totality. Prominences are the brightest objects visible continuously during the totality. They are clouds of relatively cold (10,000K) and dense matter with the same properties as that of the chromosphere matter. They emit in lines of hydrogen, helium and calcuim, which produce the pink color of prominences and the chromosphere, and can always be observed in monochromatic light.

White corona can be observed from Earth only during total solar eclipses, because its intensity is much lower than the brightness of the sky. It has several components emitting in the entire visible region of spectra. The K- (Electron or continuum) corona is due to scattering of sunlight on free high-energy electrons, which are at a temperature of 1 million degrees, and contain continuous spectra and linear polarization of the light. The K-corona dominates in the corona, have distinct 11-year cycles, and have variable structures depending on the level of solar activity. During the solar maximum, it is circular. During the solar minimum, it is symmetrical and elongated in the equatorial region, while

in the polar regions, it has bunches of short rays or plumes. During intermediate phases, it has asymmetric structure with many streamers of different lengths. The F- (Fraunhofer or Dust) corona is due to scattering of sunlight on dust particles. An F-corona has Fraunhofer spectra with absorption lines. Due to heating of dust particles close to the Sun, the F-corona evaporates, producing a large cavity in the dust distribution. An F-corona has oval shape. Its intensity decreases slowly with the distance from the Sun, and it predominates over the K-corona at long distances. The F-corona reaches near-Earth space , producing Zodiacal light (a faint conical glow extending along the ecliptic, visible after sunset or before sunrise in a dark, clear sky). The Thermal (T) corona is due to thermal emission of dust particles heated by the Sun.

Solar corona also have components emitting linear spectrum. The E- (Emission) corona is due to emission lines of highly ionized atoms of iron , nickel, and calcium. The E-corona intensity decreases rapidly with its distance from the Sun and is visible up to a 2-solar radius in monochromatic light. The S- (Sublimation) corona, was recently found, but as of 2002, its existence is still debatable. It consists of emission of low ionized atoms of Ca(II) produced by sublimation of dust particles in relatively cold parts of the corona. All these components are visible together in the corona during total eclipses.

The last and most mysterious component of the corona is giant coronal streamers observed only from the orbital coronagraph LASCO and from stratospheric flights during total eclipses. The giant coronal streamer shape and properties are

different from those of any other component of the corona. Animations of their timed development look similar to visualizations of gusts of solar wind . In the last few years, evidence has arisen demonstrating that its nature is the same as that of plasma tails of comets , fluorescence of ionized gas molecules (originated by evaporation of comets near the Sun), and is due to interaction with the solar wind and sunlight. This component of the corona is called Fluorescent (Fl) corona, but this hypothesis needs further scientific verification. The corona is divided arbitrarily to Internal corona (up to 1.3 radius), which can be observed any time by coronagraph, Medium (1.3-2.3 radius), and External corona (over 2.3 radius) where F-corona dominates. Edges of the corona gradually disappear in the background of the sky. Therefore, the size of the corona greatly depends on the spectral region of observations and clearness of the sky.

Lunar eclipses occur when the Moon passes into Earth's shadow. The Moon does not normally disappear completely; its disc is illuminated by light scattered by the Earth's atmosphere. Color of the lunar eclipse depends highly on the composition of the atmosphere (amount of ozone and dust). The full shadow (umbra) cast by Earth is surrounded by a region of partial shadow, called the penumbra. Some lunar eclipses are visible only as penumbral, other as partial. The length of the Moon's path through the umbra, divided by the Moon's diameter, defines the magnitude of a lunar eclipse.

See also Coronal ejections and magnetic storms

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eclipse

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

eclipse (ēklĬps´, Ĭ–) [Gr.,=failing], in astronomy, partial or total obscuring of one celestial body by the shadow of another. Best known are the lunar eclipses, which occur when the earth blocks the sun's light from the moon, and solar eclipses, occurring when the moon blocks the sun's light from a small portion of the earth. Occasionally a double or binary star system is aligned so that one star eclipses the other as seen from the earth; these stars are known as eclipsing binaries. Also important to science have been the eclipses of Jupiter's satellites; in 1675 the Danish astronomer Ole Roemer used these eclipses to calculate the speed of light. Observations of starlight passing near the sun during the 1919 solar eclipse were of particular value in validating Einstein's general theory of relativity.

Lunar Eclipses

Since the earth and moon shine only by the reflected light of the sun, each casts a shadow into space in the direction away from the sun. The shadow consists of a cone-shaped area of darkness called the umbra, where all light from the sun is cut off, and a larger area of partial darkness called the penumbra, which surrounds the umbra and receives light from a part of the sun's disk. Lunar eclipses can occur only when the moon is in its full phase, i.e., when the earth is between the sun and the moon. These eclipses may be total or partial, depending on whether the moon passes completely into the umbra of the earth's shadow or remains partly in the penumbra. Since the moon cuts the umbra close to the base, it can experience long periods of total eclipse ranging up to 1 hr, 42 min. A partial eclipse (when it passes through the penumbra) can last more than 2 hr, and the entire lunar eclipse may continue for as long as 4 hr. Some light is refracted, or bent, by the earth's atmosphere into the umbra, so that the moon at totality, instead of appearing black, ranges from a dull gray to a coppery color, depending on the amount of dust in the earth's atmosphere.

Solar Eclipses

A total solar eclipse can occur only when the moon is in its new phase. At this time the moon is between the sun and the earth and cannot be seen until it moves across the sun's disk. At the onset of totality, parts of the sun may be seen shining brightly between the high points of the moon's irregular edge, a phenomenon known as Baily's beads; the disk of the moon appears black and is surrounded by the sun's corona, out of which shoot immense, flamelike spurts called prominences. The sky darkens to twilight, the brightest stars become visible, and there is a noticeable drop in temperature. Baily's beads are seen again as the sun reappears and the sky grows lighter.

At apogee (when the moon is at its farthest point from the earth) the umbra of its shadow is too short to reach the earth's surface, causing the apparent diameter of the sun's disk to be larger than that of the moon. Where the moon would otherwise block the sun entirely, now the sun is seen as a bright ring completely surrounding the moon's disk; this eclipse is known as an annular, or ring, eclipse. The longest possible duration of totality for a solar eclipse is 7 min, 40 sec at or near the equator when the sun is directly overhead; the duration decreases with increasing latitude. The eclipse of June 20, 1955, lasted 7 min, 8 sec, which was the longest duration of totality in 1,238 years; an eclipse almost as long occurred on July 11, 1991.

Frequency and Prediction of Eclipses

If the plane of the moon's orbit about the earth coincided with that of the earth about the sun, a solar eclipse would be observed each month when the moon is new and a lunar eclipse when the moon is full. However, the moon's orbital plane is tilted at an angle of about 5°10′ to the earth's orbital plane, making eclipses possible only when the three bodies are aligned (at new or full moon) and when the moon is crossing the earth's orbital plane (at a point called the node). Within a given year, a maximum of seven eclipses can occur, either four solar and three lunar or five solar and two lunar. Despite the fact that there are more solar than lunar eclipses each year, over time many more lunar eclipses are seen at any single location on earth than solar eclipses. This occurs because a lunar eclipse can be seen from the entire half of the earth facing the moon at that time, while a solar eclipse is visible only along a narrow path on the earth's surface.

From their observations of eclipses the Chaldaeans (fl. 1000 BC–540 BC) discovered that similar eclipses of the sun recur in cycles of 18 years, 111/3 days; this cycle, called the saros, is an interval in which the sun, earth, and moon return to nearly identical relative positions. Since the orbits of the earth and moon are quite accurately known, eclipses can be predicted far in advance, both in time and location. Similar calculations can determine the time and place of past eclipses; this information is useful for dating historical events that are known to have occurred at the same time as an eclipse.

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Eclipse

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Eclipse

An eclipse refers to the complete or partial blocking of a celestial body by another body and can be used to describe a wide range of phenomena. Solar and lunar eclipses occur any time the Sun, the Moon, and Earth are all positioned in a straight line. This is an uncommon occurrence because the plane of Earth's orbit around the Sun is different than that of the plane of the Moon's orbit around Earth. Thus, the Moon is usually located just above or below the imaginary plane of Earth's orbit.

Although the Sun is 400 times larger the Moon, the Moon is 400 times closer to Earth. Thus, when the Moon's orbit takes it in front of Earth, it blocks the Sun from view, creating a solar eclipse. During a lunar eclipse, the opposite happens: Earth passes between the Sun and the Moon, casting a shadow on the Moon. A solar eclipse is visible only during the day, while a lunar eclipse is visible only at night. Lunar eclipses are more common and last longer than solar eclipses, and can be viewed from everywhere on the planet at night.

An eclipse may be partial, total, or annular (where one object covers all but the outer rim of another); and it may be barely noticeable or quite spectacular. The planes of Earth's orbit and the Moon's orbit

coincide only twice a year, signaling an eclipse season. Only during a small percentage of eclipse seasons do total eclipses occur.

Solar eclipses

During a solar eclipse, the Moon's shadow sweeps across Earth. The shadow has two parts: the dark, central part called the umbra, and the lighter region surrounding the umbra called the penumbra. Those people standing in a region covered by the umbra witness a total eclipse; those in the penumbra see only a partial eclipse.

Words to Know

Corona: The outermost atmospheric layer of the Sun.

Lunar eclipse: Occurs when Earth passes between the Sun and the Moon, casting a shadow on the Moon.

Solar eclipse: Occurs when the Moon passes between Earth and the Sun, casting a shadow on Earth.

Umbra: From Latin, meaning "shadow"; the completely dark portion of the shadow cast by Earth, the Moon, or other celestial body during an eclipse.

The type of solar eclipse depends on the distance of the Moon from Earth. The Moon's orbit, like Earth's, is elliptical (oval-shaped). At some points along its orbit, the Moon is closer to Earth than at others. In order for a total eclipse to occur, with the umbra reaching Earth, the Moon must be at a close point on its orbit. If the Moon is too far away, it appears smaller than the Sun and one of two things may happen. First, only the penumbra may reach Earth, creating a partial eclipse. The other possibility is that the Moon will appear to be centered within the Sun. When this occurs, a ring of brilliant sunlight, like a ring of fire, appears around the rim of the Moon. This is known as an annular (ring) eclipse.

The first stage of a solar eclipse, when the Moon just begins to cover one edge of the Sun, is called first contact. As the Moon shifts across the Sun's face, the sky begins to darken. At the same time, bands of light and dark called shadow bands race across the ground. Just before second contact, when the Moon completely blocks out the Sun, a final flash of light can be seen at the edge of the Sun, an effect called the diamond ring.

Then, at totality, all sunlight is blocked, the sky turns dark, and the planets and brighter stars are visible. During this period, the Sun's corona, or outer atmosphere, is visible as a halo. The weak light given off by the corona (about half the light of a full moon) is normally not visible because it is overpowered by the light of the Sun's surface. Prominences, jets of gas that leap from the Sun's surface, are also visible during the total eclipse. After a few minutes, the Moon begins to pass to the other side of the Sun, signaling an end to the solar eclipse.

Lunar eclipses

A lunar eclipse can occur only when the Moon lies behind Earth, opposite the Sun, and is fully illuminated. As the Moon crosses into Earth's umbra, it does not become totally hidden. The reason is that gas molecules in Earth's atmosphere refract or bend the Sun's light around the surface of the planet, allowing some of it to reach the Moon. Because the wavelengths of red light are refracted less, the Moon will appear various shades of red during a lunar eclipse.

If the entire Moon falls within the umbra, the result is a total lunar eclipse. If only part of the Moon passes through the umbra, or if it only passes through the penumbra, a partial lunar eclipse occurs. A partial lunar eclipse may be difficult to detect since the Moon dims only slightly.

[See alsoCalendar; Moon; Sun ]

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eclipse

eclipse In astronomy, partial or total obscuration of the light from one celestial body as it passes through the shadow cast by another body. Eclipses are transitory; the most familiar are lunar and solar eclipses. Within any given year, a maximum of seven eclipses can occur, either four solar and three lunar or five solar and two lunar. A solar eclipse occurs when the Moon passes between the Earth and the Sun, blocking the Sun's light from the part of the Earth on which the Moon's shadow falls. It can only happen at new Moon. The maximum duration of a total solar eclipse is 7min 8sec. A lunar eclipse occurs when the Earth intervenes between the Sun and the Moon, blocking the Sun's light from the Moon. It can only happen at full Moon. The longest duration of a lunar eclipse is 1hr 42min. Because the Earth and Moon shine only by the reflected light of the Sun, each casts a shadow into space in the direction away from the Sun. The shadow consists of a region of total darkness (umbra) and partial darkness (penumbra).

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eclipse

e·clipse
/ iˈklips/
•
n.
an obscuring of the light from one celestial body by the passage of another between it and the observer or between it and its source of illumination:
an eclipse of the sun. ∎ fig.
a loss of significance, power, or prominence in relation to another person or thing:
the election result marked the eclipse of the traditional right and center.•
v. [tr.] (often be eclipsed)
(of a celestial body) obscure the light from or to (another celestial body):
as the last piece of the sun was eclipsed by the moon. ∎
deprive (someone or something) of significance, power, or prominence:
the state of the economy has eclipsed the environment as the main issue.PHRASES:in eclipse
losing or having lost significance, power, or prominence:
his political power was in eclipse.

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Eclipse

Eclipse a famous racehorse of the 18th century and one of the ancestors of all thoroughbred racehorses throughout the world. The Eclipse Stakes, run annually at Sandown Park near London since 1886, is named in the horse's honour. (See also Copenhagen.)

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eclipse

eclipse The partial or complete obscuration of one heavenly body by another, as perceived by an observer on one of the bodies. The proper description of an eclipse also refers to the period of time involved.

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